读取应该只在启动时读取的配置文件，并将它们封装在Singleton中。

只读单例存储一些全局状态(用户语言、帮助文件路径、应用程序路径)是合理的。使用单例控制业务逻辑时要小心——单例几乎总是以多例告终

使用单例的方法之一是覆盖一个实例，其中必须有一个“代理”控制对资源的访问。单例在日志记录器中很好，因为它们代理访问，比如说，一个文件，这个文件只能被写入。对于像日志这样的东西，它们提供了一种方法来抽象出对日志文件之类的东西的写操作——你可以将缓存机制包装到你的单例中，等等……

也可以考虑这样一种情况，你有一个应用程序，有许多窗口/线程等，但它需要一个单一的通信点。我曾经使用它来控制我希望应用程序启动的作业。单例程序负责将作业序列化，并将它们的状态显示给程序中其他感兴趣的部分。在这种情况下，你可以把单例对象看作是在应用程序中运行的“服务器”类……HTH

Singleton类的主要目的是限制创建的实例数量，从而确保对资源的访问控制。

使用单例类不会产生内存空间浪费，因为它限制了实例的创建。因为对象的创建只会发生一次，而不是每次新请求时都创建。

单例类在内部使用互斥量，因此使其线程安全。这就是为什么多线程和数据库应用程序主要使用Java中的Singleton模式进行缓存、日志记录、线程池、配置设置等等

Singleton pattern is the most pervasive pattern in the Spring containerization approach. If we look at that in terms of architectural primitives - they form a blackboard graph of objects, to which every thread can read and write. They do the dramatic act of synchronizing between multiple threads. The very reason why multiple threads need to synchronize is because there are always resources that underlie a computational program, over which contention might occur. Consider what is called a 'last seat problem'. A flight is being booked, but there are multiple ways to do it. For simplicity lets assume that the data about the flight occupancy is stored in a flat file rather than a database. Now, if there are two threads, each functionally different (i.e represented by different endpoints in the webapp) and let one of these threads A, be the thread which a prospective passenger uses to make a booking and the other one B is which a flight manager uses to close the booking - virtually closing the boarding door. Then, if these threads do not use singleton, the flight object would be detached from the real resource out-there, which we say not the actual aeroplane but the entry in the flat file. The A thread would have reference to an object, while the passenger is still fighting a dilemma whether to fly or not and then finally when he makes up his mind, the B thread would already have closed the door. But the object referenced by the A thread would still show one more seat to go. Now, cutting out the RDBMS due to our initial assumption, the system would write a ticket for the passenger and issue it to him eventhough the boarding is closed. Now, in a singleton implementation, the moment the theread B accesses the system, the universal object Flight is updated with status closed. Hence, if the passenger finally makes up his mind and clicks confirm, he would get an error right away. All this would not have been possible without the singleton. Hence, singleton allows you to stay close to the resources and avoids thread contention.

您可以在实现状态模式时使用单例(以GoF书中所示的方式)。这是因为具体的State类没有自己的状态，而是根据上下文类执行它们的操作。

你也可以让抽象工厂成为一个单例。